Since the late 1950's, airborne electronic warfare antenna systems have generally utilized the broadband spiral antenna, both of the planar and conical types. The virtue of these antennas are many, particularly the planar type. First, the planar is flush mountable on an aircraft skin, thereby reducing aerodynamic loading. Second, both types demonstrate excellent and near-constant electrical performance over very broad frequency ranges. Third, an array of these antennas can be made to operate autonomously as direction finders. Accordingly, these types of antennas have served well over the years. However, they possess a serious and possibly critical shortcoming in that they can only respond to one sense of circular polarization.
There has been, in recent years, strong evidence that more and more signals of interest are being circularly polarized and, are being switched during operation from one sense to the other. Thus, electronic warfare systems antennas designed for one polarization sense will be effectively blind to the oppositely polarized emitter. Currently, the only solution is the doubling of antenna assets, further increasing space and weight loading on the aircraft.
In the prior art, many attempts have been made to address the major drawback in two and four arms spiral antennas, which is that they are conventionally fed at the center of the structure, so that only one sense of circular polarization is produced. Various attempts at achieving dual circular polarization with a spiral have met without practical success. For example, one method attempts to obtain both senses of circular polarization at the same time by feeding the antenna at both ends of the spiral arms. As expected, the pattern produced by feeding the spiral at its center is satisfactory. However, feeding the spiral at the opposite ends where the arms are normally terminated leads to many problems such as spurious radiation from higher order nodes, excessive losses, poor impedance characteristics and loss of important phase and amplitude tracking qualities. Due to inherently poor performance characteristics and limited bandwidths, this configuration has been largely abandoned as a useful concept.
Another technique is directed to the use of multi-arm (six or more) configurations which use a complex feed network to produce excitations for modes higher than is normally used or desired. One version utilizing this concept generates different mode excitations at the feed points of a six arm spiral. Patterns produced by these modes are oppositely polarized from the normally used mode patterns and possess similar attributes. This approach is expensive to implement, especially for reasonable frequency bandwidths. Further, the antenna must be relatively large (6 wavelengths in circumference at the lowest operating frequency) and losses are extremely high due to the long distance the energy must travel to reach the radiation bands. Accordingly, the drawbacks of this approach are that it is too expensive, too large, too complicated, and too performance limited to have useful application.